Just off the Italian island of Giglio, an unprecedented feat of engineering is underway to raise the enormous Cost Concordia luxury liner. It was about a year and a half ago that the cruise ship sailed onto rocks in shallow water, ripping open its hull and causing it to tip over. That wreck killed 32 people and the captain is on trial for manslaughter. Now, the operation to right the ship, so it could be towed away, was delayed a few hours by thunderstorms. But it is now starting.

NPR's Sylvia Poggioli has been keeping her eye on this and joins us on the line. Sylvia, good morning.

SYLVIA POGGIOLI BYLINE: Good morning, David.

GREENE: So tell me what exactly they're trying to do with his beleaguered ship.

BYLINE: Well, what we see as the wreck is now surrounded by support boats and barges. The port has been closed to all traffic. The start was pushed back almost three hours due to a freak thunderstorm. I'm in a filing center of about 300 yards from the wreck and close to the bow. I can see the control room barge, on which engineers are directing and monitoring the effort to bring the slumped ship upright.

You know, there's no model for an undertaking of the scale. Everything is based on hypotheses and calculations. Every single second is being monitored by computers and underwater cameras. If something is off they'll try to correct it as they go along. And the whole operation is expected to last 10 or 12 hours.

GREENE: And, Sylvia, you say this is unprecedented but there is a technical term for what they're trying to do with the ship. And it's par buckling, I've read. What exactly does this mean?

BYLINE: It's an old nautical practice in which ropes were looped under sailing ships to bring them upright. It's never been used on the ship this size - 114,000 tons, more than twice the size of the Titanic. They've tied the ship to the mainland with huge chains looped under the haul. And they've built six very large platforms on which the vessel will rest it once it's upright.

You know, it's probably the biggest, most challenging and most expensive salvage operation ever undertaken. It's estimated it could top $1 billion. And once upright, divers will look for the bodies of the two people still missing: an Italian woman and an Indian man.

GREENE: Now Sylvia, going into this, there were a lot of people who were worried that this could be, really, a risky operation. I'm wondering, what could go wrong here?

BYLINE: Well, the biggest question mark was whether the ship could be pulled free from the reef. All they knew was that it was jutting into the hull by some 30 feet. There was the risk that the ship could be wrenched apart right at the start. But then the engineers announced that three hours into the operation they saw the first sign that the wreck was shifting when underwater cameras recorded water swirling in the area where the hull has been resting on the reef. They say that it has been freed from the reef, it is now resting on another area that they have prepared for it to rest. So now the full rotation is commencing.

GREENE: There was a different option here. I mean there was talk after the shipwreck that they might actually just chop the vessel up. Is there reason they decided not to go that route?

BYLINE: Italian authorities ruled that out for environmental reasons. The ship lies in the Tuscan Marine Sanctuary. It's Europe's largest, its home to huge mollusks, whales, dolphins and some of the Mediterranean's last monk seals. Environmentalists have carefully monitored the area every day since the shipwreck. Nearly all the fuel was pumped out early on.

Now there's concern for potential leakage of polluted water still inside the ship, as well as the massive quantity of food for 4200 people who were on board. For example: 11,000 eggs, 18,000 bottles of wine, and more than 24,000 pounds of fish. Of course, most of it has rotted and the engineers warn local residents there might be a terrible stench similar to rotten eggs.

The engineers have sealed all the portholes and accessible cracks. But furniture and stuff could spill out from the damaged side as it rises; so large, absorbent, floating booms and nets are positioned all around the wreck area.

GREENE: Sounds like quite a scene there. We've been talking to and they are Sylvia Poggioli about efforts to raise the Cost Concordia. That's the big cruise line that crashed about a year and a half ago, killing 32 people.